Can't open urls in existing private window anymore

For years I'm using an external RSS reader, which opens it's URLs in the systems default browser. Until latest update, opera opened the URLs in the active, last used window. If the last used opera window was a private window, the URLs opened in this private window.

With latest Update to Opera 45.0 (Windows 10 64-bit), this doesn't work anymore. URLs always open in a normal window, even if the last active window is a private window. If only a private window exists (all normal windows are closed), a new normal window is opened for the new URL.

It was a very important feature for me, to open the URLs from the RSS reader directly in the active private window. Do you have any idea, why this doesn't work anymore and how I can fix it?